Support Logical Resources

This document outlines the state of the Eclipse Platform Support Logical
Resources plan item. Interested parties should review this document and
verify that their use cases are reflected in the requirements then later that
the solution provided satisfies their needs. Feedback is strongly encouraged and
may be provided on the platform-vcm-dev mailing list or in the bug report for this plan item.

This is the plan item:

"The Eclipse Platform supports a strong physical view of projects,
files, and folders in the workspace. However, there are many situations where
a physical view is not the most salient or useful for many purposes. In some
cases, multiple distinct objects happen to be stored in a single file, like
an archive. Conversely, in other cases, something that is logically a single
object is stored across multiple files. This discrepancy between logical and
physical creates problems for common operations such as searching, comparing,
and versioning, which need to work in the physical realm. Eclipse should support
some way of mapping between a logical view and the physical organization of
files on disk."

Last Modified: July 2nd, 2003

Motivation

By definition Eclipse supports logical resources. It's a "an open extensible
IDE for anything and nothing in particular". The applications that are
built with it can be tailored to a particular problem domain and the workbench
extensions can be used to provide support for the logical model. What Eclipse
really doesn't support is integrating tools at the logical model. It's simple,
to integrate two plugins into Eclipse either plugin A and B have to know about
each other, or both plugins have to depend on some common plugin. While the
first option is satisfactory for a few cases, the second case is the most common
and the most problematic. To illustrate, the Eclipse SDK contains several plugins
that are based on the Resources plugin (e.g. Team, Search, Compare, JDT). When
an application is packaged based on the SDK, because the plugins in the SDK
are resource based it isn't possible to integrate plugins at the logical level.

As you will see later, even the users of the SDK (JDT+CVS+Resources) can observe
some of the problems associated with the lack of support for logical resources
and the implicit mapping to physical resources. The lack of a logical integration
point causes the co-existance of plugins in Eclipse to be error prone for the
user. The next sections will examine the relationships between logical and physical
resources and outline the visible problems with Eclipse 2.1.

Existing Bugs

Logical Resources

Whereas logical resources are elements of the application that model the data/processes
of a particular problem domain, the physical resources are the file system resources
(e.g. files and folders) into which an application will eventually transform
their application model for storage. Applications that don't use file system
resources are not being considered as part of this plan item. For example, an
application that stores it's application data directly into a database.

The following table enumerates concrete examples of how logical and physical
resources are used in current applications integrated into Eclipse.

Table 1: Example logical to physical uses

Relationships

Examples

logical (1) <-> (*) physical

one logical to many physical

(1a) EJB maps to several physical files:
(WSAD + XDE Tester). This is the most common case we have found.

Note: With 1 to many relationship there is always the complication
that arises if the logical element maps to files in an other project? This
would cause many problems in the current Team plugin.

logical (*) <-> (1) physical

many logical to one physical

(2a) Methods in a class map to their
containing class file. (JDT)

(2b) Archive files (zip, jar) can be seen as a logical
and the contents of the archive all map to the same physical.(JTD + all)

logical (1) <-> (1) physical

one logical to one physical

(3a) A Java Package maps to a folder
but a package does not include sub folders (e.g. they are shallow). (JDT)

(3b) A UML class maps to a file. For example, in a UML
application a class could be stored in a single file called 'x435dsds.umlclass'.
The reason it's a logical is that the user of the application will never
be concerned about the actual file name or even the folder it's stored
in. (XDE)

Not very adaptable

Object contributions and decorators are key mechanisms for providing rich
integration between tools. Using object contributions and adaptable objects,
any plugin
can provide actions and decorators that will show in another plugin's view.

Since IResource is the lowest common integration class and
is strongly mapped to its physical storage as files and folders, the integration
point for actions and decorators cannot be the logical types in the view.
The actions will consequently use the adaptable type from the selection
to perform the
action or the decorator the resource from the getImage request.

Here is an example to illustrate the problem with the CVS plugin. When running
a CVS commit action in the packages view the action doesn't know that the IFolder
is a Java package and is a shallow namespace. It will use the selection and
traverse the selected IContainer using IContainer.members() and commit all child
resources. This is the same when the CVS outgoing decorator is shown on a package.
It will reflect the deep outgoing state of the IContainer. This can lead to
unexpected behavior because the user expected the action to run on the model
that the action was displayed on.

Decorators

When the CVS decorator decorates a logical container in a view, the desire
is to decorate parents with the dirty indicator if one or more of their logical
children are dirty. To do this properly, the decorator would need to query the
dirty status of each of the logical children of the container. This can be a
costly operation as each child may itself be a logical container. We have optimized
the dirty decoration by caching folder dirty states. This optimization becomes
far more complicated with logical models where the caching must be based on
this alternate model (i.e. the state must be cached for each model).

In addition, often decorators rely on IResource change
events to re-calculate some new state. In the case of a
logical elements representing several physical resources there is no easy way of associated
a IResource change event to its files and the actual logical element.

Views

It would be nice to see the Java logical view in the Synchronize view or the
CVS Repositories View. However, the entries in these views may not be IResources
so an IResource independent mapping mechanism would be required. This could
be a considerable amount if work for anyone providing a logical model. This
problem is partly being addressed by the introduction of customizable content
providers as described in the Generic
Navigator item.

Also if an action prompts the user, for example to add resources to CVS, what
icons and labels are used in the dialog? It would be nice to show those that
are most familiar to the user.

Change Sets

Consider a user's action as a logical change, for example, the user renames
a class using the JDT refactoring action. In the user's mind he 'renamed a class'
and as a result there has been many other file changes. Currently there is not
way of capturing these logical actions and associating the set of affected resources.
This problem manifests itself more in tools where almost every change to the
model has side effects.

File based tools

Some tools that integrate into Eclipse are file based and don't use IResources.
And thus wouldn't be able to operation logical elements.

Proposed Solution

Part of the solution is to introduce a logical resource element that can be
the basis for integration between plugins. Our proposed solution will attempt
to solve the most common uses described in Table1, namely
examples 1a, 1b and 3a.
The generic
navigator plan item should address the 2b and 3b
examples by providing extensions that would allow a logical model to be available
to other plugins for display purposes. Finally, 2a is currently
solved with the existing IResource adapter mechanism.

Naturally another possible solution would be to change IResource to not be
analogous to file system resources. However this scope of change is beyond the
3.0 timeframe and would cause, in my opinion, non-justifiable breaking changes
to all plugins.

ILogicalResource

An interface will be added to the Resources plugin that will allow binding
logical elements to physical ones. The interface is purposely simple with logical
model manipulations ommitted. A client can't use this interface to display logical
models or gain any interesting additional knowledge about it. In fact, the only
way that a plugin will be able to gain access to the model for those purposes,
would be via the navigator extensions proposed in the generic
nagivator plan item.

Current plugins that add object contributions to IResource (CVS, Search, Compare)
would have to also add them to ILogicalResource and honour the biddings provided
by the logical resource. Moreover, the plugins that adapted to IResource to
these object contributions shown in the context menu should no longer adapt
the IResource and instead implement ILogicalResource.

/** * A logical resource is an element of an application that
models the * data/processes of a particular problem domain. Eventually a
logical resource * must transform the application model for storage using a
file system
* resource. * This class provides the bridge between a logical element and
the physical * resource(s) into which it is stored. On the other hand it
doesn't provide * hierarchical access to the
logical
model. * *
@see
IResource */ public interface ILogicalResource {
/**
* Returns the project which contains this logical resource.
* This is a resource handle operation; neither the resource
* nor the resulting project need exist.
*
*
* @return the project handle
*/
public IProject getProject();
/**
* Returns the file system resource(s) for this logical resources. The
* returned resources must be contained within the same project.
* A logical element should return the same list of resources regardless
* of the existance of the files on the file system. For example, if a
* logical resource called "form" maps to "/p1/form.xml" and "/p1/form.java"
* then whether form.xml or form.java existed, they should be returned
* by this method.
*
* The traversal of the returned IResources should be done considering
* the isDeep flag. If a logical resource maps to a folder
* then the folder's child resources can only be visited if the folder is
* deep.
*
*/
public IResource[] getPhysicalResources();
/**
* Optimization check for tools that should not run operations that traverse
* into child folders.
* @return
*/
public boolean isDeep();
}

Issues

- ObjectContributions have to be duplicated in the plugins that would like
to provide both contributions for IResource and ILogicalResource.

- Logical classes (e.g. java model, UML) have to implement the interface instead
of adapting to it. This is a limitation of the current objectContribution story
in the workbench.

- Plugins that add object contributions will have to modify their actions to
handle logical resources. There should be some standard UI components for showing
the mappings. For instance you could show the logical model at the top and in
a details part the physical files/folders that will be affected by the operation.